1. The Field of the Invention
The present invention generally relates to systems and methods for producing vertical cavity surface emitting lasers (VCSELs). The invention is also related to utilizing combinations of nitrogen (N), aluminum (Al), antimony (Sb), phosphorous (P), indium (In), gallium (Ga) and/or arsenide (As) as a material system and as a means to increase VCSEL device wavelength longer than 1200 nanometers (nm) using ordinary MOCVD or MBE equipment. The present invention more particularly relates to development of a VCSEL using migration enhanced epitaxy (MEE) in processing InGaAs, InGaAsN, InGaAsNSb, GaAsNSb AlGaAsNSb and/or other combinations.
2. The Relevant Technology
Solid-state semiconductor lasers are important devices in applications such as optoelectronic communication systems and high-speed printing systems. There has been an increased interest in VCSELs although edge emitting lasers are currently used in the vast majority of applications. A reason for growing interest in VCSELs is that edge emitting lasers produce a beam with a large angular divergence, making efficient collection of the emitted beam more difficult. Furthermore, edge emitting lasers should not be tested until the wafer is cleaved into individual devices, the edges of which form the mirror facets of each device. By contrast, not only does the beam of a VCSEL have a small angular divergence, a VCSEL emits light normal to the surface of the wafer. Additionally, because VCSELs generally incorporate mirrors monolithically in their design, they allow for on-wafer testing and the fabrication of one-dimensional or two-dimensional laser arrays.
VCSELs may be made by growing several layers on a substrate material. VCSELs include a first mirrored stack, formed on the substrate by semiconductor manufacturing techniques, an active region, formed on top of the first mirrored stack, and a second mirrored stack formed on top of the active region. By providing a first contact on top of the second mirrored stack, and a second contact on the backside of the substrate, a current is forced through the active region, thus driving the VCSEL.
The active region is further made up of one or more quantum wells sandwiched between two spacer cladding regions. Inside the spacers, the active region is sandwiched by confining layers. The confining layers or regions are used to provide electrical confinement of minority carriers. By selecting the appropriate materials for the quantum well, the confining layers, and the barrier layers, a VCSEL generally may be grown or fabricated that generates light at a desirable, predetermined wavelength. For example, by using InGaAs quantum wells on GaAs substrates, longer wavelength VCSELs may be produced. The use of InGaAs quantum wells, however, causes strain in the quantum wells. If the quantum wells are grown past their critical thickness, they may relax by forming dislocations, and thus a poor quality active region results.
VSCELs may be made with GaAs and emit light in the 850 nanometer range. Because the quantum well for the short wavelength 850 nanometer VCSELs is made from GaAs (the same material as the substrate) the various epitaxially deposited layers, whose thickness is related to wavelength, are able to maintain the minimal mechanical strain without mechanical relaxation. If one were to use InGaAs in the active region at the larger 1.3 μm wavelength device range (e.g., 1200-1650 nm), however, the lattice mismatch is generally such that large layers would tend to relax their strains and suffer dislocations, produce slip lines or develop island growth, which would interfere with proper lasing.
In order to go to the proper bandgap for what is referred to in the art as a 1.3 μm wavelength (i.e., greater than 1200 nm) semiconductor lasers, one generally uses InGaAs, GaAsSb or some combination thereof instead of GaAs in the active layer. Indium gallium arsenide (InGaAs) and gallium arsenide antimonide (GaAsSb), however, do not possess the same lattice constant as GaAs at the compositions useful for 1.3 micron lasers. This makes it very difficult to build a proper quantum well structure.
The thickness of the various layers in the active region while not arbitrary have some flexibility within the constraints of the design and the process. The combined thickness of the spacers, the confining layers, the barriers and the active regions sandwiched by the mirrors must be such that a Fabry-Perot resonator is formed. The quantum wells should generally be positioned so that they are roughly centered at an antinode of the optical electric field. These two requirements define the spacer thickness in terms of the other layer thicknesses.
The barrier layer thicknesses between the quantum wells need to be thick enough to adequately define the quantum wells, but thin enough that the quantum well positions are not excessively far from the antinode of the electric field. The thickness of the barrier layers at the boundaries of the quantum well regions have some flexibility. Optimally they need to be at least thick enough that the energy levels of each of the quantum wells are nominally the same. They may be thicker if material quality issues require this.
The thickness of the quantum well is related by quantum mechanics to the well and barrier compositions, the desired emission wavelength, and the density of states. With a higher density of states narrower quantum wells may be optimally used.
Long wavelength quantum wells are a challenge to construct. The semiconductor laser, e.g., VCSEL, structure needs a design to achieve long wavelength quantum wells normally fabricated on GaAs substrates. It is therefore very desirable to have a quantum well (i.e., the active layer (or quantum well layer) and the barrier layers surrounding the active layer) making use of materials such as GaAs, InGaAs or GaAsSb in the construction of a VCSEL operational above the 1200 nm range. It may be advantageous to remedy the foregoing and other deficiencies in conventional devices and to facilitate the production of longer wavelength VCSELs by introducing migration enhanced epitaxy (MEE) into the VCSEL fabrication process.
Therefore, systems and methods may be developed here utilizing MEE during molecular beam epitaxy (MBE) growth of quantum wells used in semiconductor lasing devices such as VCSELs. Methods and systems shown here may be used for producing semiconductor lasers exhibiting enhanced quantum well performance.